reloadData not working until I tap my screen in iOS in Core Data and NSFetchedResultsController

Question

Here's my current situation:

1. When I open up my app, UITableView on my first screen is blank

2. I fetch my data from a remote server and parse its resultant JSON object, and save it to Core Data storage, and issue a fetched request to the storage and show the result to the aforementioned UITableView

3. However, even after I post a notification after the parse-save-query and reload my table, the table isn't updated until I tap or scroll over the screen.

I tried [_myTableView reloadData]; and [_myTableView performSelectorOnMainThread:@selector(reloadData) withObject:nil waitUntilDone:NO]; from notification, but the result doesn't change at all and I still have to tap or scroll over the screen in order to see the reloaded data on the table.

I use Core Data and NSFetchedResultsController, and I confirmed my query was successfully fetched. Just a reload table issue. And also note that I call the notification from AppDelegate.m but my actual table view is on RootViewController.m, another class, for the reasons stated above (remote server data fetching).

Also, when I wrote out the following code:

-(void)controllerDidChangeContent:(NSFetchedResultsController *)controller {
    [_myTableView reloadData];
    [_myTableView endUpdates];
}

I still have to see the result by tapping the screen. However, when I added the following code:

-(void)controllerWillChangeContent:(NSFetchedResultsController *)controller {
    NSLog(@"updated begin");
    [_myTableView beginUpdates];
}

I cannot see the reloaded data even after I tap the screen at all.

Also, using printf debug, I confirmed that even if the two methods above are called many, many times (probably it's called in the same number of times of the actual fetched managed objects, right?), the numberOfRowsInSection: and cellForRowAtIndexPath: aren't called at all after I fetched the newly added datasets.

Note that cellForRowsAtIndexPath: is called and the data is displayed if I wait for 10 or 20 seconds or so, which means it's displayed even if I don't tap or scroll over the screen. Weird.

So what's happening here? And how can I fix this strange issue?

I use iOS 7 and Xcode 5.

update

I found out that the reason I wasn't able to see the reloaded data when I write controllerWillChangeContext: is I got the following error: CoreData: error: Serious application error. An exception was caught from the delegate of NSFetchedResultsController during a call to -controllerDidChangeContent:. Invalid update: invalid number of rows in section 0. The number of rows contained in an existing section after the update (1) must be equal to the number of rows contained in that section before the update (0), plus or minus the number of rows inserted or deleted from that section (0 inserted, 0 deleted) and plus or minus the number of rows moved into or out of that section (0 moved in, 0 moved out). with userInfo (null). It's still quite weird that I don't get this error when I implement controllerDidChangeContext: only, but if I write both, I got the error and the table doesn't show the data even after I tap the screen.

So maybe I cannot take the route of displaying blank result first, and updating the table with the new data, and instead I should just make users wait to show the correct data... ?

Answer 1

Calling reloadData in controllerDidChangeContent: is a wrong approach. When you implement a NSFetchedResultsControllerDelegate the delegate is responsible for all table view updates.

---

The import from your remote server should be done in a background queue. Before merging your background context with your main context via mergeChangesFromContextDidSaveNotification, you should

1. Disable the NSFetchedResultsControllerDelegate.
2. Merge your changes with the main context.
3. Reload your table.
4. Re-Enable the NSFetchedResultsControllerDelegate.

Sample:

fetchedResultsController.delegate = nil;
[mainContext mergeChangesFromContextDidSaveNotification:notification];
[tableView reloadData];
fetchedResultsController.delegate = yourController;

---

Hint: When using the NSFetchedResultsControllerDelegate you have to implement at least:

• controllerWillChangeContent:
• controller:didChangeObject:atIndexPath:forChangeType:newIndexPath:
• controllerDidChangeContent:

Read the docs for a sample implementation: NSFetchedResultsControllerDelegate

Answer 2

Here is some sample code

Loading Data in the Background

First implement the 3 main fetchedResultsControllerDelegate methods correctly to ensure your UITableView gets updates correctly.

• controllerWillChangeContent:
• controller:didChangeObject:atIndexPath:forChangeType:newIndexPath:
• controllerDidChangeContent:

Don't call [tableView reload] from within these delegate methods use only the Apple sample code to correctly add/remove/update rows in the tableView.

1. Create background thread Create managedObjectContext on backgroundthread (bgContext)
2. Register observer for save notifications on the bgContext (call method storesDidSave)
3. Run background load/delete job save in batches
4. Process save notifications
5. On completion of background job remove observer of bgContext save notifications

You can put this code anywhere, as long as you have access to the MAIN managedObjectContext - probably in the object that is running the background load jobs. MAKE SURE YOUR ARE USING THE CORRECT THREAD TO PROCESS. ManagedObjectContext's are not thread safe!

// NB - this may be called from a background thread so make sure we always run on the main thread !!
// This is when transaction logs are loaded
- (void)storesDidSave:(NSNotification*)notification {

    // Ignore any notifications from the main thread because we only need to merge data
    // loaded from other threads.
    if ([NSThread isMainThread]) {
        FLOG(@" main thread saved context ignore");
        return;
    }

    // Must pass these to the  main thread to process
    [[NSOperationQueue mainQueue] addOperationWithBlock:^ {
        FLOG(@"storesDidSave ");

        // Now merge into the MAIN managedObjectContext and save it.
        // If you don't save then objects in the main context don't seem to update correctly,        
        // especially deletes seem to don't show
        if (self.managedObjectContext) {
            [self.managedObjectContext mergeChangesFromContextDidSaveNotification:notification];
                    NSError *error;
            FLOG(@" saving moc");
            if (![self.managedObjectContext save:&error]) {
                FLOG(@" error saving context, %@, %@", error, error.userInfo);
            }
        }
    }];
}

// Sample background dispatcher
- (void)loadDataInBackground {

    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^(void) {
        [self loadData];
    });

}
// Background load job calls save after batches of 100 records
- (void)loadData {
    FLOG(@"loadData called");
    _loadJobCount++;
    [self postJobStartedNotification];

    FLOG(@" waiting 5 seconds...");
    sleep(5);
    [self showBackgroundTaskActive];

    NSManagedObjectContext *bgContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSConfinementConcurrencyType];

    // Register for saves in order to merge any data from background threads
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(storesDidSave:) name: NSManagedObjectContextDidSaveNotification object:bgContext];


    while (self.persistentStoreCoordinator == nil) {
        FLOG(@" persistentStoreCoordinator = nil, waiting 5 seconds to try again...");
        sleep(5);
    }

    bgContext.persistentStoreCoordinator = [self persistentStoreCoordinator];

    FLOG(@" starting load...");

    for (int i = 1; i<=100; i++) {
        // Add a few companies
        [self insertNewCompany:bgContext count:10];
        [bgContext processPendingChanges];

        // Save the context.
        NSError *error = nil;
        if (![bgContext save:&error]) {
            FLOG(@"  Unresolved error %@, %@", error, [error userInfo]);
        }
        FLOG(@"   waiting 2 seconds...");
        sleep(0.01);
    }

    // Register for saves in order to merge any data from background threads
    [[NSNotificationCenter defaultCenter] removeObserver:self name: NSManagedObjectContextDidSaveNotification object:bgContext];

    FLOG(@" loading ended...");
    [self showBackgroundTaskInactive];

    sleep(2);
    _loadJobCount--;
    [self postJobDoneNotification];
}